Conventional actuators, sometimes referred to as "motors", have a movably supported member, and a coil. When a current is passed through the coil, a motive force is exerted on the member. A control circuit is coupled to the coil in order to controllably supply current to the coil. One example of such an arrangement is found in a hard disk drive, where the movable member of the actuator supports a read/write head adjacent a rotating magnetic disk for approximately radial movement of the head relative to the disk. There are situations in which it is desirable to move the member to one end of its path of travel at a predetermined velocity which is less than its maximum velocity. An example of such a situation is a power failure. In such a situation, it is desirable to move the member to a parking location, where it is held against potentially damaging movement which could
When a current is applied to the coil of the actuator, the member is subjected to a force tending to accelerate the member at a rate defined by the magnitude of the current, and in a direction defined by the polarity of the current. Consequently, in order to accelerate or decelerate the member until it is moving at a desired velocity and in a desired direction, it is important to know the actual direction and velocity of the member. In this regard, it is known that the back-EMF voltage on the coil of the actuator is representative of the velocity and direction of movement of the member. Specifically, the following relationship applies to actuators: EQU V.sub.M =I.sub.M *R.sub.M +K.sub.e .omega.
where:
V.sub.M =voltage across actuator (motor), PA1 I.sub.M =current through actuator, PA1 R.sub.M =internal resistance of actuator, PA1 K.sub.e =torque constant of actuator, and PA1 .omega.=velocity of actuator. The term, K.sub.e .omega., represents the back-EMF of the actuator coil.
It is desired to have a control circuit for an actuator that accurately monitors the back-EMF of the actuator coil and effectively controls the movement of the actuator member under widely varying load conditions, so as to, for example, permit the control parking of a disk drive member throughout the path of its travel. Further, it is desired to have such a control circuit that is implemented with a minimum of circuitry so as to maintain the incremental cost of such a control circuit at an acceptably low level.